1. Field of the Invention
The present invention relates generally to an information recording medium, and more particularly, to an information recording medium having an information recording thin film whereon information can be erasably recorded.
2. Description of the Prior Art
An optical information recording medium records information by changing the optical state or minute surface shape of a thin film formed on a substrate. The changes are made by focusing laser light on the surface of the thin film in order to raise the temperature of the selected parts of the surface.
A non-erasable optical disc which uses a thin film of a metal or a thin film containing a low oxide of a metal as the host material has been reported in J. Appl. Phys., Vol. 54, No. 9, September, 1983. According to this report, information is recorded by utilizing the increase in optical density in the portion irradiated by a laser light to raise the temperature on a thin film formed by vapor evaporation on a substrate. The reliability of such an information recording thin film, especially in the case of leaving it in a high temperature atmosphere, has a high correlation with the thermal transition temperature of thin information recording film. The transition temperature is defined as follows. The optical state of the information recording thin film changes at a specified temperature when the information recording film is heated gradually from a low temperature to a high temperature. The lowest of the particular temperatures at which the above-mentioned change in optical state takes place is defined as the transition temperature of the information recording thin film. To measure the transition temperature, a low power laser light beam is irradiated on the information recording thin film and the temperature of the film is gradually raised at a constant rate while observing the transmitted light and reflected light at the same time (Japanese Patent Application Sho 59-70229). The temperature of various information recording thin films are measured by using this prior art method of measurement, adn then a reliability test is also performed, and the experiments show that the information recording thin films having a transition temperature of above 120.degree. C. did not deteriorate even when left alone for several months in a high temperature atmosphere of 50.degree. C.
Another conventional information recording thin film comprises TeO.sub.x (O&lt;x&lt;2) as the host material and further contains Sn, In, Ge, Bi or Se as an additive. This information recording thin film is known to have the property of reversibly changing its optical state depending on the condition of the irradiation of the laser lights, as disclosed in the Japanese Unexamined Published Patent Application Sho 56-145530 and Japanese Patent Application Sho 58-58158). When a pulse light of relatively short duration is irradiated on the thin film, the temperature of the irradiated portion rises once and then quickly drops, thereby decreasing the optical constants of the irradiated portion. This is defined as whitening. In contrast, when the thin film is irradiated by a relatively low power pulse light of long duration, the irradiated portion is heated once to a higher temperature and then gradually cooled, thereby increasing the optical constants. This is defined as darkening. This type of information recording media is used by utilizing the above-described whitening and darkening phenomena so that writing and erasing are repeatedly carried out. That is, erasable recording is possible, and application of the whitening and darkening phenomena to optical information recording discs is being developed.
When the above-described phenomena are applied to an optical information recording disc, it has been shown experimentally that whitening (i.e., decreasing the optical constants) is done in a shorter time period than is darkening (i.e., increasing the optical constants). As a result, in the actual recording of signals, whitening is used to record the signal, in order to attain a higher recording rate and recording density, while darkening is preferably used to erase the signal. Accordingly, in an information recording disc having the above-described thin film, the thin film must be preliminarily set in the darkened state, that is, the state having larger optical constants, prior to the recording of signals.
Hereinafter, when the information recording thin film is in a virgin state, that is, when no heat treatment or the like has yet been carried out thereon, that state will be defined as the initial state, and the above-mentioned states of high optical constants and low optical constants will be defined as the darkened state and the whitened state.
The inventors carried out assessment tests of reliability under a high temperature atmosphere for recording media having erasable information recording thin film. The assessment shows that reliabilities in the initial state are substantially the same for various transition temperatures. However, the experimental assessment tests show that with regard to the recorded portion, i.e., the portion in the whitened state, reliability has no particular correlation with respect to the transition temperature; that is, even though the transition temperatures are the same, reliabilities vary depending on the composition of the thin film and elements added thereto. In other words, in the whitened state, reliability becomes unsatisfactory, even though the transition temperature is sufficiently high.